See the colonist Durnin who is still milking him until he collapses.
Save file:
https://mega.nz/#!guIlXAqR!Dcxe_nGyfeZNQMU_UTXmfWZO72Wn0mBmFC_xxv1_ulw
On it, thanks for the report!
(If you're curious, the root bug was that the muffalo is attempting to nuzzle the colonist, and they're repeatedly interrupting each other.)